Dinoflagellate Factories: The Dino Life History

Dinoflagellates undergo a somewhat complex life cycle. Under normal
circumstances they reproduces just like any cell, by asexual division
-- new dinos pinch off from old ones. But under certain conditions, usually
stressful ones like cold, starvation, no light, etc., they will do something
different. Two dinoflagellates will join together (called fusion). This
stage is the planozygote stage. Then they change, sometimes quite dramatically,
in shape and size. They get much bigger and they harden their shell (sometimes
forming the spikes in the drawing.) Their insides change too, adding a
lot of fat and oil, not unlike a bear getting ready to hibernate. In fact,
dinoflagellates often seem to "hibernate" for the winter. When
they are all done changing, the new, fat dinoflagellate is called a cyst
or a hypnozygote. Then when the weather warms up, the cyst breaks out
of its shell, forming a temporary stage called the planomeiocyte (not
present in all species). This stage quickly reforms its shell (remember
the theca?) and becomes a normal dinoflagellate
again.